Default value of *some* fields in a mutable struct

General Usage
4

This seems like a great use case for a macro. You might take inspiration from https://github.com/mauro3/Parameters.jl which does something similar:

julia> using Parameters

julia> @with_kw struct Foo
         x::Int32 = 1
         y::Float64
       end
Foo

julia> Foo(y = 2)
Foo
  x: Int32 1                                                                                                                                                                                                                                                                                                                          
  y: Float64 2.0

Note that @with_kw doesn’t give the behavior you’re looking for. Instead, it throws an error if any field without a default is not provided:

julia> Foo()
ERROR: Field 'y' has no default, supply it with keyword.

But that’s an intentional design decision, and you could write something similar to @with_kw that would do exactly what you want. There is also related work in @kwdef which is supplied with Julia base:

julia> Base.@kwdef struct Bar
         x::Int32 = 1
         y::Float64
       end
Bar

julia> Bar(y = 2)
Bar(1, 2.0)

julia> Bar()
ERROR: UndefKeywordError: keyword argument y not assigned
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